lxd/storage/backend/lxd: Use new instance for creation date in CreateInstanceFromMigration
lxd/storage/backend/lxd: Use new instance for creation date in CreateInstanceFromMigration

And use source snapshots for creation date of target snapshots.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd